Radar guided air to air missiles
currently represent the best of what state of the art technology can
offer, both in terms of range, accuracy and resistance to
countermeasures. This reflects in the fact, that these weapons are only
used by the world's frontline air forces, the maintenance of the
complex fire control systems required being beyond the abilities of the
average Third World country. In comparison with the Western World, even
the Warpac air forces use few of these weapons, up to the mid seventies
only the USSR using a number of types on air defence aircraft of the IA
PVO-Strany. However, the situation is changing, as the Russians are
currently equipping tactical aircraft with radar guided versions of the
AA-7 and AA-8 and low level penetration will become more difficult, for
Western interdiction aircraft as the new Super Foxbat, with its
lookdown shoot-down capable 25 nm AA-X-9, or rather AA-9, is deployed.

On the brighter side, ... a competitive shoot-off ended
between Hughes and Raytheon for the Amraam (Advanced Medium Range
Air-Air Missile), Hughes winning the contract. Amraam is the
replacement for the Western air forces' radar guided Sparrow. The
weapon is a fire and forget, active radar guided missile with inertial
midcourse guidance, enabling launches against targets pursuing the
launch aircraft.

With a range and speed better than the Sparrow, this overall
capability is packaged into an airframe comparable in size to the IR
Sidewinder, allowing the F-14 and F-15 to carry eight of these weapons,
instead of the customary four radar guided weapons. Some reports also
indicate that the late eighties Sidewinder replacement, the ASRAAM, may
also be fitted with active radar guidance, in preference to the IR
guidance of its predecessor.

Radar guidance systems detect and home in on their targets by
sensing electromagnetic energy reflected from the target's surface. The
source of the reflected radiation is a radar transmitter; in the
instance of weapons with active radar guidance, this transmitter is
situated within the missile; in the case of semiactive guidance, it is
carried by the launch aircraft. In either case the transmitter must
beam electromagnetic radiation at the target, this radiation must
travel to the target, reflect, travel back to the receiving antenna of
the missile, be amplified, demodulated and analysed to determine the
direction of the target, this information then enables the guidance
computer to steer the weapon toward the target to achieve a kill. An
effective weapon must have the ability to discriminate between the
target's return and reflections from its background, i.e. the surface
of the Earth or ocean, it should also be capable of resisting jamming
or deception and be able to penetrate through adverse weather
conditions.

Radar

Radar theory is an extremely complex subject requiring a good
understanding of electromagnetism, and wave theory, fortunately though,
the basic principles are fairly straightforward.

Electromagnetic waves are generated whenever we induce
changes, typically oscillations, in an electric or magnetic field.
These waves then propagate outward at the speed of light, 3.108 msec '.
The rate at which the oscillation occurs then determines the
wavelength, by the relationship lambda
= c/f ( lambda = wavelength, c = velocity of light, f =
frequency
of oscillation).

For practical purposes, if we intend to create directional
means of these waves, we must employ a wavelength shorter than the
dimensions of our antenna (an antenna being a device which radiates or
receives electromagnetic waves), current radar applications involving
wavelengths of the order of a metre down to centimetres, these
corresponding to frequencies from 1 GHz (109 cycles/sec) to around 60
GHz (classified as the microwave band).

The term radar is an acronym - Radio Detection And Ranging. A
radar is comprised of two basic subsystems - a transmitter and a
receiver. A transmitter is a device which generates a microwave signal,
this signal is usually modulated (typically pulsed on-off), amplified
and fed into a transmitting antenna. As compared to low frequency
electromagnetic energy, microwaves cannot be conducted by conventional
cables, they require waveguides (waveguides are hollow [rectangular or
circular] sections with inner walls coated with conducted layers - the
common term used is plumbing), these must have extremely low losses
because the power output of the transmitter is usually of the order of
Kilowatts (or tens to hundreds of kW in pulsed applications).

Pulsed outputs are used for a number of reasons, the main
factors being rangefinding and peak power output. The range of a target
can be easily determined by measuring the time it takes for a pulse to
travel from the transmitter to the target and back. In considering the
power output, the more power delivered = the greater the range and
ability to resist jamming, on the other hand the larger the demands on
the transmitter's main output amplifier (or oscillator).

The solution is found in pulsing the output, the time between
the pulses being much longer than the duration of the pulse (consider a
peak output of 100 kW, pulses 10 msecs long 100 msecs apart - the
average power output is only 10 kW). The output power is then fed into
an antenna, which focusses it into a beam. Surveillance radars usually
employ fairly wide beams, the objective being the detection of the
target, tracking beams, on the other hand, must be very narrow, as they
serve to accurately measure the position of the target with respect to
the radar.

Antennas may be conventional parabolic dishes or in newer
systems, phased arrays, which may scan electronically without the need
to point the antenna. The transmitted microwave energy then propagates
through the atmosphere toward the target. Like all forms of
electromagnetic radiation, microwaves are attenuated by the atmosphere
- both absorbed and scattered. Scattering is primarily due to water
particles in the atmosphere, however, as the wavelength of the
radiation is much larger than the size of the water droplets,
microwaves do not experience the catastrophic attenuation IR does (see
IR guidance, March 1982), though the effective range will be decreased
as the amount of water present increases.

Absorption is a quantum physical effect (TE March 1982), in
the instance of microwave wavelengths this is mainly due to resonance
in the Oz molecule, which exhibits absorption lines between 30 and 0.5
cm. Over larger distances this may cause a reasonably large loss of
signal. The energy which covers the distance between the source and
target then experiences absorption and reflection on the target's
surface.

Exposing its belly, this
F-14A
displays the three classes of missile it is armed with - IR heat
seeking, semi-active radar and active radar. The semi-active AIM-7F
Sparrow (starboard glove pylon) is a late model of the AIM-7 used
during the Vietnam war (at the time plagued by low reliability), the
weapon has a maximum range around 100 km, cruising speed Mach 4 and
carries a 40 kg continuous rod warhead. This missile will equip the
RAAF's F-18A fighters, though it will be later replaced by the smaller
and more capable Amraam. The large weapon beneath the fuselage is the
AIM-54A Phoenix, with no doubt the world's most lethal air-to-air
missile, with a range of 200 km and a big 60 kg warhead. The current A
version will be shortly replaced with the newer AIM-54C, equipped with
more capable digital signal processors and with a lighter and cheaper
airframe. (Lcdr. Dave Erickson, VF-51, USS Kitty Hawk)

Electrically conductive materials usually reflect very well,
sharp straight edges on an airframe often behave like antennas, in
general curved surfaces are worse reflectors than flat surfaces
(consider the shape of the B-1, which has 1/10 the radar cross-section
of a B-52). A usual measure of an aircraft's ability to reflect
microwaves is its radar cross-section (12.566 power reflected per unit
solid angle/power incident on target), which varies with the direction
of the incident radiation. A fighter, head-on, has a cross section
between 0.1 and 1 m2 for the 3 to 10 cm band, whereas a bomber could
approach 10 mz (don't try thinking of a B-52's cross section!). The use
of composite materials reduces the signature, just as radar absorbing
paints help.

The reflected microwave energy then travels back to the
receiver, which in many instances employs the same antenna as the
transmitter. The signal which reaches the receiver is a mixture of a
target return, reflected energy from the background (clutter) and
electrical noise. Depending on the type of receiver it may or may not
be amplified, after which it is mixed with a microwave signal of a
higher frequency, a process known as superheterodyning.

Mixing creates sum and difference frequencies, the difference
frequency being in the high frequency (tens to a hundred MHz) band,
this frequency is then amplified (due to a number of reasons, it is
difficult to directly amplify microwave signals) and subsequently
demodulated. The demodulated signal is then processed by electronics to
yield information on the target, typically range and velocity relative
to the radar.

Modern radars employ complex techniques to reject clutter,
employing high speed digital signal processors, these also serve to
circumvent jamming or deception.

Semi-active Radar Guided AAMs

Semi-active radar guided missiles dominate the World's radar
guided missile population, basically by virtue of their relative
simplicity. The vast majority of currently operational designs
originated in the 1950s, aside from a number of Soviet types, recently
deployed (they did have some catching up to do). The AIM-7 Sparrow is a
development of a 1950s weapon, just as the Skyflash is, in turn, a
development of the AIM-7 itself. In the mid-fifties, when it was
decided to develop radar guidance for air-air missiles, it was
impossible, with state of the art technology, to package a radar
transmitter and receiver of the appropriate range into a medium sized
missile. Were it possible to fit all the systems in, the abysmally low
reliability of vacuum tube electronics would make the operational
deployment of such a weapon rather a hindrance than a gain to an air
force's combat capability.

As things turned out, two different guidance systems were
tried, beam riding guidance and semi-active guidance. The former class
is generally regarded as extinct (in a beam riding system, the missile
travels along a tracking beam transmitted by the launch aircraft's fire
control radar). The weapon's accuracy is given only by the fire
control's tracking accuracy, which need not be very good, particularly
at long ranges. This, and problems associated with the dynamics of the
weapons flight, led to the eventual demise of the whole class (AIM-7A,
AA-1 Alkali). Currently, beam riding (laser, though) is used by the
RBS-70 SAM. The latter class of weapon not only survived two decades,
in fact it thrived and currently represents the main medium range AAM
in most frontline air forces.

In a semi-active guidance system, the launch aircraft
acquires
the target with its fire control radar, and if the conditions are
right, will track it. The Weapons Systems Officer (F-4, typically) will
then power up the missile and lock the launch aircraft's illuminator
onto the target. The illuminator is usually a small, separate narrow
beam radar transmitter which can be selectively pointed at a target by
use of the tracking information generated by the fire control radar. If
the missile's guidance then succeeds in locking on to the target's
radar return, the missile may then be launched.

The AIM-7, as carried by the F-4, F-14, F-15, F-18 is ejected
from its mount and when clear from the launch aircraft, fires its solid
propellant rocket engine. It then accelerates to its cruise velocity,
pointing itself at the target. The guidance system will generate an
error signal if the weapon points at anything else than the centre of
the target's radar cross section. Most weapons employ proportional
navigation, due to the nature of the guidance, this allows for all
aspect, typically head-on kills.

When the target is within the lethal radius of the weapon's
warhead, a proximity fuse, usually radar, detonates the warhead,
commonly a high explosive/fragmentation type (the timing of the fuse is
critical, an Israeli F-4E failed to kill a Syrian Foxbat in a head-on,
snapup AIM-7 attack simply because the missile, fused for targets
travelling at transonic speeds, detonated after passing the Mach 3 MiG,
failing to cause any damage) and destroys the target. Most weapons have
miss distances of the order of metres, though the Skyflash has
apparently narrowed that down to the order of a metre. The most
important factor determining a semi-active guided weapon's lethality is
its tracking accuracy and ability to discriminate between the target's
return and ground clutter.

Earlier weapons employed conical scan seekers, however newer
systems rather use monopulse seekers, as these are more accurate and
resist jamming better, though at the expense of added complexity.

Conical Scan
Semi-active
Seekers

Conically scanning seekers established themselves very early,
as the dominant type of seeker in use, this was basically due to their
conceptual simplicity and undemanding signal processing electronics,
easily implemented with vacuum tubes. The principal element in the
system is a rotating antenna. This antenna, usually a dish, rotates
about the axis of the missile, however, the axis of the antenna (the
axis of its main lobe - an antenna lobe being the pattern in space we
would get, if we moved around the antenna with an electromagnetic field
strength meter and marked all the points with an equal field strength)
is offset by several degrees, when the antenna rotates, its axis draws
a cone around the missile axis. (See diagram 1).

When the launch aircraft is illuminating the target, it
behaves, as far as the seeker is concerned, as a point source of
electromagnetic energy. If the target lies within the seeker's cone,
rotating the antenna will modulate the output signal leaving the
antenna, as the signal is stronger as the target is closer to the axis
of the antenna lobe.

This results in a sinusoidal variation in the amplitude of
the
output signal. The direction of the target can be found from the phase
of the modulation, with respect to the direction of the antenna
relative to the missile's axis. The variation in amplitude contains the
information as to the other angular component of the target's
direction. Simple phase and amplitude detectors can readily extract
error signals, these can then be fed into the missile's guidance
computer, which can accordingly determine the correct control surface
deflection for optimal guidance to the target.

The signal transmitted by the illuminator may be pulsed or
continuous-wave (CW), a pulsed signal offers higher peak outputs but
may be more readily jammed. In its basic configuration, a conically
scanned system may be easily jammed, providing we do know the rate at
which it rotates. If we transmit a signal at the same frequency as the
illuminator, but amplitude-modulate it with a frequency very close to
the frequency of the seeker's antenna rotation, we will succeed in
creating a false error signal, which will throw the missile off course.
Apparently USN F-4Ss of the USS Midway experienced some problems during
joint manouevres with the RAN, as its seems, the AIM-7 seekers could
not digest chopped returns reflected by the rotating props on the RAN's
S-2s. Conical scanning is not likely to be used in any future designs,
as it is being displaced by monopulse seekers.

Monopulse Semiactive Seekers

Monopulse seekers derive all target bearing information from
a
single pulse, i.e. a continuous wave illuminating signal. These seekers
are very demanding in the stability of the system's electronics and
require compact, high gain receivers, all of these factors making
vacuum tube implementation very difficult; on the other hand, they are
highly resistant to amplitude modulation jamming - as a result of these
factors, it was only in the 1970s that monopulse systems saw
operational deployment, typically the British Aerospace Skyflash.

A phase comparison monopulse system (see diagram 2) utilizes
phase differences (time lags) between incoming signals to generate
guidance error signals. If the target lies along the missile's axis,
the target return enters each receiver simultaneously. However, if the
target lies off axis, the return will enter the receiver on the side
closer to it earlier, i.e. it will have a phase lead over the return
entering the other receiver.

This phase difference is proportional (for small errors) to
the error angle between the target and missile axis and may be easily
detected by the electronics. On the other hand, though, any drift in
the receivers which could alter the signal's phase during processing
would generate a false error signal. A practical system would employ
four receivers, two for each axis. Each of these two receivers would
drive a phase detector, which would generate the given error signals.
These would be subsequently fed into computers, to find the required
control deflections.

Monopulse systems, such as the Skyflash seeker, are very
accurate and resist jamming. Good clutter rejection allows snap-down
attacks on targets as low as 250 ft, test trials of the Skyflash were
very successful, with several direct impact kills.

Active Radar Guided AAMs

Active radar guided missiles are the Rolls Royce of the
air-to-air missile world. Probably the most extreme example of what
they are capable of, is the Hughes AIM-54 Phoenix. Launched from the
F-14, the weapon is targeted by the large AWG-9 radar and fire control
system of the launch aircraft. The missile has a cruise velocity in the
region of Mach 5 (note: classified), after covering a distance of up to
100 nm it will destroy its target with a 60 kg warhead; a note of
interest - in a number of trials AIM-54s with dummy warheads destroyed
drones by direct impact.

Active radar guidance has, up to date, been restricted only
to
large weapons, as the added complexity of a transmitter and its
associated systems made it impossible to fit into a medium or small
sized weapon.

Even so, the limited amount of available space has had one
very noticeable effect on the weapon configuration - only relatively
small transmitters are used, their limited power output enabling only
short range operation. If it were possible to output adequate power,
another problem would probably arise - the small diameter of the
missile would limit the size of the antenna used, accurate information
as to the bearing of the target would call for as long an antenna as
possible. These factors would severely limit the range of this class of
weapon, however a number of means exist to eliminate this problem - all
providing mid-course guidance, leaving the active radar guidance for
the terminal homing phase of the weapon's flight.

The first option is command link guidance. In this instance
the launch vehicle's or site's radar would accurately track the target
and launched missile, a computer would find the required flightpath
corrections for the missile, which would then be transmitted via a data
link to the missile's flight control system. When in range for an
effective lock on with the onboard radar, the weapon would initiate its
terminal guidance phase using its own radar and computer, no longer
requiring guidance commands. This type of system is often used in
surface-to-air missile systems.

Another option available is the use of inertial mid-course
guidance. The weapon is equipped with a radar and an inertial reference
system (typically a 3 axis gyroscopic device - the Amraam is to use a
strapdown gyro). Just prior to launch, the fire control computer will
provide the missile's computer with the target's position and the
parameters of its flightpath. Using the inertial system to continuously
track its own position, the missile will follow a flightpath which will
bring it within radar range of the target. The weapon will then switch
on its own radar, locate the target, lock on, home in and destroy it.
This system has one great advantage - the target need not know of the
approaching missile until it's too late, complemented by the fact that
it is not possible to jam a gyro, as compared to a command data link or
tracking/illuminating beam. Another advantage offered is the
possibility of multiple launches at independent targets, eg. up to six
Amraams may be launched close to simultaneously, at individual targets.

The third option one may choose is the use of semi-active
radar midcourse guidance. Like in all semi-active radar systems, the
fire control employs a microwave beam to illuminate the target. The
missile receives this energy and uses it to guide in within the range
of its own radar, which is then used for the terminal phase.
Semi-active midcourse guidance offers the advantage of simplicity, as
the missile need only use its own radar in a passive mode, without any
datalink receivers or inertial reference systems. On the other hand,
though, this form of guidance lends itself to deception and jamming, if
appropriate measures are not taken.

Active radar guidance is likely to become far more common in
the future, as high power microwave solid state devices are perfected,
enabling the construction of compact and reliable transmitters. Faster
and more capable microprocessor chips (or even faster bit-slice
processors) will give the weapon itself a better ability to resist
jamming and discriminate between targets and clutter. Powerful signal
processors would allow the guidance itself to take over many of the
functions currently handled by the launch aircraft's fire control, such
as resolving individual targets in a formation. The missile could
operate in a captive search mode, prior to launch, circumventing the
need to use the launch aircraft's fire control radar - ultimately the
weapon could be carried by non-radar equipped aircraft, in the same
fashion as current fire-and-forget IR AAMs. With the trend toward
smaller and lighter fighter aircraft, this becomes even more
attractive, the simpler the fighter's radar/firecontrol, the greater
the reliability and hence availability for combat.

Radar missile guidance offers both range and adverse weather
operation which cannot be matched by IR or optical guidance. One may
assume that future designs, rather than utilising a single form of
guidance, with its tightly confined launch envelopes, would use a
combination of sensors, which could make jamming and/or deception
difficult, if not impossible, in practical situations. The ultimate
goal may be seen as a small, compact, all aspect, all weather, all
altitude, short and long range, fire-and-forget weapon, most likely to
materialise in the late 1990s, if the high energy laser doesn't get
there first.